Apparatus for spinal fusion using implanted devices

ABSTRACT

A spinal implant replaces excised tissue removed during spine surgery. This implant includes fasteners which firmly attach it to vertebrae adjacent to excised tissue so as to transmit tension and torsional loads to and from those vertebrae. The body of the implant has through cavities into which bone growth material is placed during surgery. The body of the implant also has a finite modulus of elasticity in compression so as to share compressive loads with emplaced bone growth material and with new bony growth facilitated by the emplaced material and the load sharing.

This application is a continuation-in-part of application Ser. No.09/072,777, filed May 6, 1998 (pending).

BACKGROUND OF INVENTION

1. Field of Invention

This invention relates generally to the treatment of injured,degenerated, or diseased tissue in the human spine, for example,intervertebral discs and vertebrae themselves. It further relates to theremoval of damaged tissue and to the stabilization of the remainingspine by fusion to one another of at least two vertebrae adjacent ornearly adjacent to the space left by the surgical removal of tissue.More particularly, this invention relates to the implantation of deviceswhich can be inserted to take the structural place of removed discs andvertebrae during healing while simultaneously sharing compressive loadto facilitate bony fusion by bone growth between adjacent vertebrae toreplace permanently the structural contribution of the removed tissue.This invention further relates to the implantation of devices which donot interfere with the natural lordosis of the spinal column. Thisinvention further relates to implants which are radiolucent to permitmore accurate diagnostic imaging follow up.

2. Background of the Invention

For many years a treatment, often a treatment of last resort, forserious back problems has been spinal fusion surgery. Disc surgery, forexample, typically requires removal of a portion or all of anintervertebral disc. Such removal, of course, necessitates replacementof the structural contribution of the removed disc. The most commonsites for such surgery, namely those locations where body weight mostconcentrates its load, are the lumbar discs in the L1-2, L2-3, L3-4,L4-5, and L5-S1 intervertebral spaces. In addition, other injuries andconditions, such as tumor of the spine, may require removal not only ofthe disc but of all or part of one or more vertebrae, creating an evengreater need to replace the structural contribution of the removedtissue. Also, a number of degenerative diseases and other conditionssuch as scoliosis require correction of the relative orientation ofvertebrae by surgery and fusion.

In current day practice, a surgeon will use one or more procedurescurrently known in the art to fuse remaining adjacent spinal vertebraetogether in order to replace the structural contribution of the affectedsegment of the disc-vertebrae system. In general for spinal fusions asignificant portion of the intervertebral disk is removed, and ifnecessary portions of vertebrae, and a stabilizing element, frequentlyincluding bone graft material, is packed in the intervertebral space. Inparallel with the bone graft material, typically additional externalstabilizing instrumentation and devices are applied, in one method aseries of pedicle screws and conformable metal rods. The purpose ofthese devices, among other things, is to prevent shifting andimpingement of the vertebrae on the spinal nerve column. These bonegraft implants and pedicle screws and rods, however, often do notprovide enough stability to restrict relative motion between the twovertebrae while the bone grows together to fuse the adjacent vertebrae.

Results from conventional methods of attempting spinal fusion have beendistinctly mixed. For example, the posterior surgical approach to thespine has often been used in the past for conditions such as scoliosis,using Harrington rods and hooks to align and stabilize the spinalcolumn. In recent years many surgeons have adopted anterior fusionbecause of the drawbacks of the posterior approach, the primary problembeing that in the posterior approach the spine surgeon must navigatepast the spinal column and its nerve structure. However, results ofanterior surgery are variable and uncertain because constraining thevertebrae from this side does not address the loads put on the spine byhyperextension, such as from rocking the body in a backwards direction.

Pedicle screws and rods, always implanted posteriorly, tend to looseneither in the bone or at the screw-rod interface if fusion is notobtained. Fusion rates for posterolateral instrumented fusions rangefrom 50% to 90%. It must be kept in mind that plain x-rays are only65-70% accurate in determining fusion status and most studies use thisinadequate method to determine fusion status, suggesting that thenon-union rate may be greater than reported. It is also known thatposterior pedicle screw systems do not prevent all motion anteriorly,leading to the risk of fatigue failure of the metal and screw breakage.This continued motion may also lead to persistent pain, despite solidposterior bony fusion, if the disc was the original pain generator.These well documented failures of pedicle screws have given rise toextensive litigation in the United States.

In contrast to the U.S. common practice of using either IBF devices,implanted from the anterior position, or pedicle screws, implantedposterior, in Europe, spine surgeons use both IBF devices and pediclescrews in combination to achieve stability of the spine. Theseprocedures may be more successful in producing fusion but are far moreinvasive and costly and have higher morbidity for the patient.

More generally there is a great deal of variability in technique anduncertainty in outcome for the various methods now in use for spinalsurgery. For example, Fraser, R. D. points out in "Interbody, Posteriorand Combined Fusions," Spine, V20(24S):1675, Dec. 15, 1995, "[A]nalysisof the literature does not indicate that one form of fusion issignificantly better than another for degenerative conditions of thelumbar spine." Fraser did not have the results of recent studiesinvolving use of metal interbody cage devices. Ray, Charles D. reportedthe results of the original IDE study involving his Ray Threaded FusionCage (Ray-TFC) in Spine V22(6):667, Mar. 15, 1997. Two hundred eightpatients had two year follow-up and were reported to have 96% fusionrate with only 40% excellent results and 25% fair or poor results.

There are only two published reports on the use of the BAK ThreadedInterbody Fusion Cage. The first, published by Hacker, R. J., SpineV22(6):660 Mar. 15, 1997 compares posterior lumbar interbody fusionusing the BAK device to anterior and posterior fusion with allograftbone. Hacker found that patient satisfaction was equivalent but overallcosts were less for the BAK. Zucherman reported on the early experiencewith laparoscopically assisted ALIF with BAK but no outcomes data arepresented on these first 17 patients. Kuslich, S. D. presented theresults of the multi-center IDE study of 947 patients who had fusionsusing the BAK device at the 1996 annual meeting of the North AmericanSpine Society in Vancouver. He reported a fusion rate of 90.5% and somedegree of functional improvement in 93% of patients with pain eliminatedor reduced in 85.6% of patients. The data so far for these threadedcages is scanty at best. It is clear that the results are better thanthose for posterior fusion with or without pedicle screw instrumentationbut further studies are needed. Problems with threaded devices will nodoubt come to light as they are used under less controlled circumstancesin greater numbers of patients.

John Kostuick, M.D., Chief of Spine surgery at Johns Hopkins Hospital,Baltimore, Md. (Private Communication with James Nicholson, 2nd R. RoyCamille Meeting, Paris, France, Jan. 28, 1998) vigorously maintains thatfusion cannot take place within a metal IBF device which shields thebone from load. Dr. Tromanhauser, one of the inventors, in a series of30 patients implanted with BAK cages, found that at least 9 patients hadcontinued back pain with x-rays and CT scans that were inconclusive fordetermining fusion. Surgical exploration of these patients has revealedcontinued motion and no obvious fusion. All patients were explored atleast 6 months after cage implantation, a point at which most surgeonswould expect fusion.

Recent unpublished research by Dr. Elsig also indicated that 60% of thecases he reviewed had to be reoperated due to failure 6-8 months afterinitial surgery. There is therefore recognition and belief, especiallyamong Kostuick Fellows who adhere to the principles of Wolff's law, thatloading the bone during fusion through the implant device connecting theopposing remaining vertebrae would yield superior fusion both instrength and in length of healing time.

It is also well established from the study of bone growth that a bonewhich carries load, especially compressive load, tends to grow andbecome stronger. Existing stabilizing implants, in particular IBF's, donot share any of the compressive load with the new bone growth, in factpossibly shielding new bone growth from load. For example, the BAK cageis promoted as being so strong that a pair of BAK cages will support thefull body load. Such shielding is well known to inhibit new bone growthand healing, however.

The biggest limitation in any method of fusion at the present time isthe nature of available devices for bridging the space left by excisionof diseased or damaged tissue. In particular, interbody fusion (IBF)devices currently on the market in the United States do not providestability in all planes of motion. There is very little evidence tosupport the biomechanical stability of these devices. They are generallystable in compression (forward flexion) unless the bone is osteoporotic,which condition could lead to subsidence of the device into the adjacentvertebral body with loss of disc space height. They may be much lessstable in torsion and certainly less so in extension where there is noconstraint to motion except by the diseased annulus fibrosus which iskept intact to provide just such constraint. It is doubtful that adegenerative annulus could provide any long term "stiffness" and wouldmost likely exhibit the creep typically expected in suchfibro-collagenous structures.

Another problem with conventional fusion devices and with IBF's inparticular is difficulty in diagnostic follow-up. In assessing whetheror not fusion has taken place between adjacent vertebrae and inside theIBF device, normally plain x-rays including flexion and extension viewsare obtained. The usual method (Cobb) of measuring motion on thesex-rays has a 3 to 5 degree range of error, well beyond the motion thatmay be present leading to pain. It is impossible to see inside a metalIBF with plain x-rays and conclude anything about fusion status. CTscans with reformatted images are increasingly used because of theseshortcomings. Newer software for CT scanners has improved the ability to"see" within cages but the metal artifacts produced by the x-rays arestill significant and limit the conclusions that can be drawn. Drs.Tromanhauser and Kant have found virtually no differences in CT scanstaken immediately post-op and those taken at a six month follow-up.

Accordingly, there is wide spread recognition among spine surgeons ofthe need for a flexible radiolucent implant device which would replaceremoved degenerated tissue and be firmly affixed mechanically toopposing vertebrae. Such a device would dramatically increase theprobability of successful fusion because it a) would eliminate orsignificantly reduce relative movement of the adjacent vertebrae and theintervertebral fixation device in extension and torsion, b) wouldthereby reduce or eliminate the need for supplemental external fixation,c) by compressive load sharing would stimulate rapid growth of the boneelements packed within the intervertebral device by causingosteoinduction within the bone chips, thereby accelerating fusion, d)would allow confirmation that fusion had taken place using standard CTor possibly plain x-rays, and e) would have the potential to bebioabsorbable, potentially being fabricated from such materials as aD-LPLA polylactide or a remodelable type-two collagen so as to leave inthe long term no foreign matter in the intervertebral space. Inaddition, a flexible implant device can be fabricated in whole or inpart from human bone autograft or from bone allograft material which issterilized and processed, automatically approximately matching theelastic properties of the patient's bone. The success rate of fusionusing such an approach is anticipated to exceed the success rate of theIBF devices or the external fusion devices alone and at least equal thecombined success rate of the current combination IBF and posteriorinstrumented technique.

However, there is currently no known method of mechanically affixing aninterbody implant device, such as those known in the art as "cages," toadjacent vertebrae. All present IBF devices simply jack open theintervertebral space, relying on the muscle, ligamentous, and annularstructures which surround the vertebra to hold the implants in place.The annulus is always degenerative in these cases and could not possiblyfunction in any predictable way and therefore cannot be relied upon toprovide adequate motion stability.

Furthermore, prior art cages are filled with bone chips which areshielded from compressive load by the stiff metal cage, preventingnatural bone ingrowth through the porous cages because the new bonegrowth cannot be loaded through the rigid implant. This leads to lack offusion because the bone, according to Wolff's law, wants to resorb dueto stress shielding by the cages. In an effort to overcome thisphenomenon, some manufacturers are adding bone growth factors to thecage and/or the bone graft in an attempt to "fool" the bone into fusingthrough the cage. However, there is no existing method of sharingcompressive loads with bone growth material and new bone growth.

Lordosis, which is a pronounced forward curvature of the lumbar spine,is a factor which needs to be taken into account in designing lumbarimplants. It is known in the art that preserving the natural curvatureof the lumbar spine requires designing into a new device such as thecurrent invention a modest taper approximately equivalent to theeffective angularity of the removed tissue. The restoration of normalanatomy is a basic principle of all orthopedic reconstructive surgery.

Therefore there is a perceived need for a device which simultaneouslyand reliably attaches mechanically to the bony spinal segments on eitherside of the removed tissue so as to prevent relative motion in extension(tension) of the spinal segments during healing, provides spaces inwhich bone growth material can be placed to create or enhance fusion,and enables the new bony growth, and, in a gradually increasing mannerif possible, shares the spinal compressive load with the bone growthmaterial and the new growth so as to enhance bone growth andcalcification. The needed device will in some instances require a modesttaper to preserve natural lumbar spinal lordosis. It will also beextremely useful if a new device minimizes interference with orobscuring of x-ray and CT imaging of the fusing process.

Thus it is an object of the current invention to provide a stabilizingdevice for insertion in spaces created between vertebrae during spinalsurgery. It is a further object to create an implantable device forstabilizing the spine by preventing or severely limiting relative motionbetween the involved vertebrae in tension (extension) and torsionloading during healing. It is a further object to provide a device whichpromotes growth of bone between vertebrae adjacent to the space left bythe excised material by progressive sharing of the compressive load tothe bone graft inserted within the device. It is yet a further object toprovide mechanical stability between adjacent vertebrae while bone growsthrough a lumen in the implant and at the same time not diminish thenatural lordosis of the lumbar spine. It is a further object of theinvention to provide a device which avoids or minimizes interferencewith various imaging technologies. It is yet another object of thisinvention to be capable of being fabricated from human bone allograftmaterial.

SUMMARY OF THE INVENTION

The invention disclosed here is a novel implant designed to achieve theforegoing objects. The design of the new implant for spinal surgeryincludes the possibility of fabricating the device out of material whichis elastic, especially in response to compressive loads, preferably witha compressive elasticity closely matched to that of human bone,preferably the patient's bone. In particular, the design includes thecapability to fabricate the device from human bone allograft material.The design is also such that the implant mechanically fastens or locksto adjacent vertebrae and stabilizes the involved vertebrae in tensionand in torsion while transmitting a portion of the vertical compressiveload to new bone growth associated with the device. This feature of theinvention will cause osteoinduction within the bone chips loaded intothe implant and will share a sufficient portion of the load withexisting bone and with the new bone growth to promote further bonegrowth and not interfere with bone fusion growth. This invention can betapered to preserve natural lordosis. This invention also minimizesinterference with x-ray imaging by virtue of being fabricated in wholeor in part from radiolucent materials.

The implant of this invention joins two vertebrae by means of amechanical fixation device which is hollow to allow bone growth matterto be added to one or more spaces communicating with the top and bottomsurfaces for the purpose of promoting fusion. The attachment portion ofthe mechanical fixation device is, in a first embodiment, a tongue andgroove mechanical fastening arrangement. Other mechanical fastenerscommonly used in the woodworking art, such as tack and staple devices,can also be used. The mechanical properties of the device are closelymatched to the bone's modulus of elasticity so as to promoteosteoinduction and rapid bone growth. The devices are generallytransparent to existing radiologic imaging techniques so as to allowfollow up confirmation of fusion of the adjacent vertebrae. The implantcan also be fabricated from bioabsorbable materials so as to leave nolong term foreign matter in the body. Human bone allograft material canalso be used as the material from which the implant device isfabricated.

In its most general form, the invention is an implant for mechanicallyattaching to the ends of and promoting bony fusion of at least twovertebrae adjacent to a space left by surgically removed spinal tissue,comprising a load-sharing body comprising a structure having acombination of structural elements fabricated from at least one materialhaving a greater than zero elastic compliance, the combinationcomprising at least a top surface and a bottom surface; said combinationof structural elements establishing for the structure as a whole acomposite greater than zero elastic compliance at least in compressionin directions generally axial to said top surface and said bottomsurface; said combination of structural elements further comprising atleast one cavity communicating with both said top surface and saidbottom surface in a configuration suitable as a receptacle for boneimplant and growth material; and on each of said top surface and saidbottom surface at least one fastener capable of mechanically anchoringthe body to said adjacent vertebrae and thereby transmitting tensile andtorsional loads to and from said adjacent vertebrae.

In another embodiment, the invention generally is an implant formechanically attaching to the ends of and promoting bony fusion of atleast two vertebrae adjacent to a space left by surgically removedspinal tissue, comprising a structure formed from a single piece of boneallograft material and having a top and a bottom, said structure havingan internal cavity communicating with said top and said bottom forreceiving autograft bone implant material and bone growth factors, saidunitary structure having at least one dovetail tongue protrusion on eachof said top and said bottom for mechanically interlocking with saidadjacent vertebrae by forming a mechanical tongue-and-groove joint.

In yet a third embodiment, the invention is an implant for mechanicallyattaching to the ends of and promoting bony fusion of at least twovertebrae adjacent to a space left by surgically removed spinal tissue,comprising a composite structure fabricated with at least two separateportions of bone allograft material with different structural propertiesand having a top and a bottom, said structure having an internal cavitycommunicating with said top and said bottom for receiving autograft boneimplant material and bone growth factors, said unitary structure havingat least one dovetail tongue on each of said top and said bottom formechanically interlocking with said adjacent vertebrae.

An important aspect in the implant procedure is the preparation of thespace to receive the implant and the grooves for the dovetail fasteners.A cutting jig is used which distracts the vertebrae and stabilizes themduring preparation and acts as a guide for precise cutting. Specialtomes are designed to precisely cut the dovetail and prepare the endplate surface. The tomes have an offset which provides for the implantto be sized to slide through the jig but fit very tightly in the spacecut into the vertebrae so as to prevent backout of the implant. Once thecutting jig is in place an x-ray is taken to show that the end of thedistraction tangs are clearing the spinal canal. The tomes have depthstops which prevent cutting beyond the distraction tangs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a frontal view of an implant of this invention placed betweenlumbar vertebrae.

FIG. 1B is a side view of the same implant.

FIG. 2 is a plan view of the same implant.

FIG. 3A is a plan view of an implant showing cavities communicating withtop and bottom surfaces into which bone growth material is placed.

FIG. 3B is a frontal view of the same implant showing cavities.

FIG. 3C is a side view of the same implant showing cavities.

FIG. 4A shows a composite implant with inset titanium endplates in planview.

FIG. 4B is a frontal view of a composite implant with inset titaniumendplates.

FIG. 4C is a side view of a composite implant with inset titaniumendplates.

FIG. 5 is an isometric representation of the second embodiment using ahorseshoe shaped tongue and groove dovetail fastener and showing theretaining barb.

FIG. 6 shows the implant of FIG. 5 inserted between adjacent vertebrae.

FIG. 7 is an isometric view of a modular implant.

FIG. 8 is an isometric view of the same modular implant with partialdepiction of adjacent vertebrae.

FIG. 9 shows an implant with a retaining barb.

FIGS. 10A and 10B depict the handle of the emplacement instruments forpreparation of the implant site.

FIGS. 11A and 11B show further details of a cutting tool or instrumentfor preparation of the implant site.

FIGS. 12A and 12B show the operation of the interlock mechanism for thecutting instrument for preparation of the implant site.

FIGS. 13A, 13B, and 13C show the cutting instrument for preparation ofthe implant site with dovetail tome deployed.

FIGS. 14A and 14B display details of the dovetail tome.

FIG. 15 is an isometric view of the driver.

FIGS. 16A and 16B show detail of the placement implement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the currently preferred embodiments, torsional and tensionalstability of the spine are provided by fasteners comprising dovetailjoints which engage grooves cut during surgery in the vertebrae adjacentto the removed tissue such that the implant and which has large surfacecontact areas. The dovetails transfer extension and torsional loadsbetween the two vertebrae and the flat contact surface transmits thecompressive loads. The device further comprises one or more holesthrough and/or cavities inside the implant such that the spaces createdcan be filled with bone graft material which will grow into and attachto the healthy vertebral bone. Optionally in all embodiments tapers toaccommodate natural lumbar lordosis can be incorporated as necessary.

In this discussion, we use for convenience a definition of "elasticcompliance" as the elastic displacement per unit of applied force, inother words the reciprocal of stiffness. The composite elasticcompliance of the device is selected at a value which promotes sharingof compressive load with bone graft and growth material and new bonygrowth. As discussed at greater length below, in one embodiment, humanbone allograft material is used to fabricate the implant. The new fusionbone will gradually share an increasing portion of the compressive loadsexperienced by the spine because the implant is made of a material, suchas a polymer, which has a compressive modulus which works in conjunctionwith the implant design to closely match the modulus of elasticity ofbone during deformation under load. The polymer, or in one embodimenthuman bone allograft material, has the added advantage of beingtransparent in x-ray imaging permitting, easy visualization of thefusion process at the vertebral interface. In a variant of oneembodiment, metal retaining clips may be located in the implant surface,both above and below the dovetails, to engage the cortical bone andprevent the implant from migrating out of the intervertebral space. Theretainers will generally be metal in order to benchmark x-ray imagingfor locking engagement assessment. In yet another variation, lockingbarbs will be included on the implant top and bottom surfaces to assistin securing the implant to adjacent bony surfaces to minimize pullout.

In a second embodiment of the implant, a plurality of dovetailprotrusions, or a compound dovetail protrusion in the approximate layoutof a horseshoe may be located on the outboard portions of the implant,thereby utilizing the strength and rigidity of the vertebrae to supportthe spinal column load. In this case the device would contain a hollowcentral core which would be filled with bone chip and biological mediumto accelerate the fusion in the intervertebral space.

In the first preferred embodiment, as shown in FIGS. 1A and 1B(elevation views), vertebrae L4 and L5 (or vertebrae L5 and S1) aremechanically attached by the implant of this invention 3. The device 3is held mechanically to the adjacent vertebrae 1 and 2 by tongue andgroove, or dovetail, arrangements 4. As shown in FIG. 2 (plan view), theimplant 3 is sited so as to provide mechanical support to the spine bothin compression and in tension, but not so as to intrude into the space 6occupied by the spinal nerve bundle. In this preferred embodiment, asshown in FIG. 2, the implant 3 will include penetrations or holes 7 thepurpose of which is to contain bone growth material to facilitate bonyfusion of the adjacent vertebrae. The implant itself may comprise avariety of presently acceptable biocompatible materials such asPolyphenolsulfone, Polyaryletherketone (PEEK), Polysulfone, Acetal(Delrin), UHMW Polyethylene, and composites of these materials involvinghigh strength carbon fibers or REM glass filaments to add tensile andshear strength. As discussed more extensively below, the implant mayalso be fabricated from human bone allograft material, autograftmaterial, or bone substitute material, such as coral or calciumphosphate. The body of the implant may optionally have a modest taper toaccommodate the natural lordosis of the lumbar spine.

One possible problem with an implant with dovetail fasteners fabricatedfrom a material such as polysulfone is that torque on one adjacentvertebra relative to the other may place large tension stresses on theangular portions of the dovetail, thereby causing breaking and crazingof the polysulfone. Thus a variation on this embodiment comprises acomposite implant fabricated from plastic material such as polysulfonefor the body and titanium for endplates bearing the dovetailprotrusions.

FIGS. 3A, 3B, and 3C show one possible arrangement of such a compositestructure, with a titanium endplate 8 set into the plastic (andradiolucent) body 9. FIGS. 4A through 4C show a variation on thisarrangement with the endplate extending to the shoulders of the plasticbody of the implant 11. Both FIGS. 3 and 4 show a variation of thisstructure, with the titanium endplate 12 set into the plastic body ofthe implant 9 and 11 in a configuration designed to provide throughspaces or cavities 14 in which to place bone growth material. In theselatter configurations, the polysulfone body is insert molded into thetitanium endplates. The titanium dovetail fasteners possess the tensilestrength necessary to avoid fracture or crazing, but the body is still"see through" with respect to X-ray and other methods of visualizinghealing progress. In addition, holes in the titanium endplates which arealigned with the bone growth material cavities provides "see through"capability in the vertical direction for assessing new bone growth.

A second major preferred embodiment, shown in isometric view in FIG. 5,is inserted between two vertebrae, e.g., L4 and L5 or L5 and S1 andmechanically attached by two or more dovetail joints, or by a compoundhorseshoe shaped dovetail, located on each of the top and bottomsurfaces of the implant to the adjacent remaining vertebrae by acomposite tongue and groove mechanism similar to but larger than thatused to secure the implant of the previous embodiment. In thisconfiguration, the implant comprises either a horseshoe shaped dovetailtongue 33 which in effect creates two dovetail joints per surface towardthe outboard ends of the implant top and bottom surfaces or simply twooutboard dovetail tongues without the horseshoe top closure. Thehorseshoe top closure may be substantially curved or it may besubstantially straight, with relatively square corners where thedovetail tongue angles back into the body of the vertebra. In avariation on this embodiment, inside the horseshoe shaped dovetailtongue protrusion 33 the body of the implant is hollow, that is, itcontains an opening or cavity 34 communicating with both the top surfaceand the bottom surface into which bone growth material is placed.

In this preferred embodiment, as further shown in the isometric view ofFIG. 6, the implant 35 with a relatively squared off horseshoe topclosure will have a surface approximately flush with the exteriorsurface of the adjacent vertebrae and will appear to create one verywide dovetail 37. This embodiment of the implant will also includepenetrations or holes in addition to or as an alternative to that shownin FIG. 5, 34, the purpose of which is also to contain bone growthmaterial to facilitate bony fusion of the adjacent vertebrae. As in theprior configuration, the implant 35 is sited so as to provide mechanicalsupport both in compression and in tension to the spinal column, but notso as to intrude into the space 6 occupied by the spinal nerve bundle.The implant in some cases is further inserted inside remaining segmentsof intervertebral disc tissue 38. As shown in both FIGS. 5 and 6, anoptional feature of these embodiments is for the faces of the implant tohave locking barbs 36 to retain the implant in place between theremaining vertebrae once it is inserted.

This implant, as in the prior embodiment, may itself comprise a varietyof presently acceptable implant materials such as PEEK (PolyestherEsther Ketone), Acetyl (delrin), polysulfone, Ultra High MolecularWeight Polyethylene (UHMW Poly), and composites involving high strengthcarbon fibers or glass filaments to add tensile and shear strength.Again, as discussed at greater length below, human bone allograftmaterial may be used to fabricate this device. This embodiment may alsobe fabricated with a modest taper to accommodate natural lordosis.

A third preferred embodiment of the lumbar implant, shown in isometricview in FIG. 7, comprises three elements, two modular dovetail halves,41 and 42, which are inserted between vertebrae L4 and L5 or L5 and S1and mechanically attached by two dovetail protrusions (similar to thosefabricated for the second embodiment) located on the top and bottom ofthe implant to the adjacent vertebrae by a tongue and groove mechanismsimilar to but larger than that used to secure previous embodiments ofthe implant. The two modular dovetail halves are held together by aretainer 43. As in the prior configuration, as shown in the isometricview of FIG. 8, the implant 35 is sited so as to provide mechanicalsupport both in compression and in tension to the spinal column, but notso as to intrude into the space 8 occupied by the spinal nerve bundle.

In this preferred embodiment, as shown in FIG. 8, the implant 35 willinclude a cavity 39 the purpose of which is to contain bone growthmaterial to facilitate bony fusion of the adjacent vertebrae. The openspace 39 is packed with bone growth material and then capped with aretainer, 43, designed to snap in place to add stability to the implantand to retain the bone growth factor to prevent it from migrating. Thisimplant, as in the prior embodiment, may itself comprise a variety ofpresently acceptable implant materials such as PEEK (Polyesther EstherKetone), Acetyl (delrin), polysulphone, Ultra High Molecular Weightpolyethylene (UHMW Poly), and composites involving high strength carbonfibers or glass filaments to add tensile and shear strength. Again themodular dovetail halves may be tapered to accommodate lordosis.

Any of the foregoing embodiments can additionally have a feature shownin FIGS. 5, 6, and 9, namely a retractable barb 36. This barb comprisesa spring wire which when deployed engages the adjacent vertebrae toprevent the implant from dislodging. A retraction tool may be insertedinto the hole 39 to cause the sigma-shaped barb to retract itsprobe-like end so that the implant disengages from the adjacentvertebra.

As previously noted, any of the foregoing embodiments of the Cor-Lok™interlocking implant can be fabricated from cadaver bone which isprocessed to form bone allograft material. Tissue grafting of livingtissue from the same patient, including bone grafting, is well known.Tissue such as bone is removed from one part of a body (the donor site)and inserted into tissue in another (the host site) part of the same (oranother) body. With respect to living bone tissue, it has been desirablein the past to be able to remove a piece of living tissue graft materialwhich is the exact size and shape needed for the host site where it willbe implanted, but it has proved very difficult to achieve this goal.

On the other hand, processing of bone material which does not containliving tissue is becoming more and more important. Non-living bonegrafting techniques have been attempted both for autografts and forallografts. For example, Nashef U.S. Pat. No. 4,678,470 discloses amethod of creating bone graft material by machining a block of bone to aparticular shape or by pulverizing and milling it. The graft material isthen tanned with glutaraldehyde to sterilize it. This process canproduce bone plugs of a desired shape.

In the Nashef process, the process of pulverizing or milling the bonematerial destroys the structure of the bone tissue. The step of tanningit with glutaraldehyde then renders the graft material completelysterile.

In the prior art, inventors have believed that it is desirable tomaintain graft tissue in a living state during the grafting process.There is no doubt that the use of living tissue in a graft will promotebone healing, but much surgical experience has shown that healing can beachieved with allografts of non-living bone material which has beenprocessed. In fact, spine surgeons express a distinct preference forsuch materials, and at least one supplier, the MusculoskeletalTransplant Foundation (MTF), has introduced femoral ring allografts forspine surgeries.

It is now possible to obtain allograft bone which has been processed toremove all living material which could present a tissue rejectionproblem or an infection problem. Such processed material retains much ofthe structural quality of the original living bone, rendering itosteoinductive. Moreover, it can be shaped according to known and newmethods to attain enhanced structural behavior.

Research shows that such allografts are very favorable for spinalsurgery. According to Brantigan, J. W., Cunningham, B. W., Warden, K.,McAfee, P. C., and Steffee, A. D., "Compression Strength Of Donor BoneFor Posterior Lumbar Interbody Fusion," Spine, Vol. 18, No. 9, pp.12113-21 (July 1993):

Many authors have viewed donor bone as the equivalent of autologousbone. Nasca et al . . . compared spinal fusions in 62 patients withautologous bone and 90 patients with cryopreserved bone and foundsuccessful arthrodesis in 87% of autologous and 86.6% of allograftpatients. (Citations omitted.) Moreover, as previously noted, sources ofsafely processed allograft material have recently become available.

In the present invention, allograft bone is reshaped into one of theCor-Lok™ configurations for use as a spine implant. Various methods,including that of Bonutti, U.S. Pat. Nos. 5,662,710 and 5,545,222, canbe used to shape the allograft material into the desired shape.

In the first sub-embodiment of this aspect of the current invention,bone material which yields to compressive loads at the exterior surfaceswithout significant degradation of the interior structural properties,such as cancellous or trabecular bone, is shaped. It is not unusual thatreshaping of graft tissue is necessary to obtain the best possiblegraft. In particular, bone tissue may be stronger and better able tobear force when it is denser and more compact.

Compression of allograft bone is desirable from general considerations.Generally, bone samples are stronger when they are more dense.Compressing allograft bone increases its density and thus generallystrengthens the allograft. The allograft bone also stays togetherbetter. In addition, recent studies have indicated that the shell ofvertebral bone is very much like condensed trabecular bone. Mosekilde,L., "Vertebral structure and strength in vivo and in vitro," Calc.Tissue Int. 1993; 53(Suppl):121-6; Silva, M. J., Wang, C., Keaveny, T.M., and Hayes, W. C., "Direct and computed tomography thicknessmeasurements of the human lumbar vertebral shell and endplate," Bone1994; 15:409-14; Vesterby, A., Mosekilde, L., Gunderson, H. J. G., etal., "Biologically meaningful determinants of the in vitro strength oflumbar vertebrae," Bone 1991; 12:219-24. Compressing bone allograftmaterial prior to implantation thus generally produces a stronger graft.

Compression also allows conversion of larger irregular shapes into thedesirable smaller shape, thereby permitting more disparate sources ofallograft bone to be used. By compressing bone to a given shape it ispossible to configure the allograft to match a preformed donee siteprepared by using a shaped cutter to cut a precisely matching cut space.In particular, this method of formation facilitates the formation ofdovetail tongue protrusions on the upper and lower surfaces of theimplant for the formation of a tongue-and-groove mechanical joint withadjacent vertebrae.

In the current invention, a blank is cut from cancellous or trabecularallograft bone and placed in a forming apparatus. The forming apparatuscompresses the sample into the desired shape. In particular, thisprocess forms the dovetail tongue protrusions on the implant upper andlower surfaces for the tongue-and-groove joint. The cancellous ortrabecular material yields at the external surface under the pressure toform a compacted layer around the outside of the allograft form. Thiscompacted layer is not destroyed material but rather forms substantiallya structure with properties of the vertebral shell or of a monococquedesign, including additional structural properties such as enhancedtensile strength. This enhanced tensile strength enables the allograftmaterial to perform the same function in resisting torsion and extensionof the spine as does the synthetic materials previously discussed. Suchprocesses in general are able to maintain the homologous property of theallograft material.

In the second embodiment of this aspect of the current invention,different types of allograft bone are formed into a composite structureto provide the necessary structural properties. Both cortical or shelland dense cancellous or trabecular bone may be compacted into a unifiedstructure. Fibrin "glue" is highly suitable for use as an adhesive insuch structures. Fibrin is a blood component important in bloodclotting. It can be separated or centrifuged from blood and has thenature of an adhesive gel. Fibrin can be used as an adhesive, either ina natural state or after being compressed, to hold together materialsuch as separate tissue pieces pressed together in a tissue press. Inparticular, cortical bone from the same source can be used as a shell toprovide needed additional structural properties, such as tensilestrength to a composite shape. Cortical bone can also be provided in ashell, much like the known femoral ring implants, to provide the neededstructural properties. Moreover, a shell is not the only structuralelement which can be added in this way. Buttresses, gussets,cross-braces, and other structural elements can be included in the sameway. Using such materials, the homologous property of the bone allograftmaterial may be maintained.

In another sub-embodiment, a relatively thin external shell of asynthetic material can be provided for enclosing compressed allograftmaterial and providing any needed additional structual properties. Afterthe graft is compressed, the shell is placed around the graft. The shellmay be made of a material which expands after it is placed in the spine,thereby supplementing the interlocking properties of the Cor-Lok™mechanical design by improving the fill between the allograft and thedonee site. There are a number of suitable materials which expand whenthey come in contact with water or other fluids. One is PEEK(polyether-etherketone)(water absorption about 1.5%). A desiccatedbiodegradable material or suitable desiccated allograft material mayalso be used.

The expansion can take place in one of two ways. First, the retainer canitself be compressed, as with the tissue, then expand when placed in thebody. Second, the retainer can be made of a material which expands whenit comes in contact with water or other bodily fluids.

It should be noted that the entire allograft implant can itself becompressed so that it expands when contacted by water. The expandableshell material can first be compressed with the allograft material,which then expands when placed in the body.

It should further be understood that the graft can be multiple tissuefragments rather than a composite material. The compressing process canbe used to compress multiple bone fragments into one larger piece. Itshould also be understood that the compression process can be used toadd additional materials to an allograft composite. For example, to bonetissue there can be added tri-calcium phosphate, an antibiotic,hydroxyapatite, autografts, or polymeric materials.

FIGS. 10A through 16B depict the surgical tools used to install theimplant. This apparatus comprises a set of unique tools which willaccurately cut a dovetail joint in bone for the purpose of inserting animplant which locks adjacent vertebrae together.

The guide 44, shown in FIGS. 10A and 10B, is a tubular tool with tangs45 extending from one end. The tangs, tapered 46 to conform to naturallordosis, are inserted between the vertebrae 47 and distract them to apreferred dimension 48, as shown in FIG. 10B. The driver 68, shown inFIG. 15, can be used with a rod extension guide adapter 70, also shownin FIG. 15, to drive the guide 44 into place. This step establishes afixed reference relative to the two vertebrae 47 and secures thevertebrae from moving. The length 49 of the tangs 45 is consistent withthe other tools in the set and establishes the extent 49 to which anytool can penetrate. A lateral x-ray is used to assure that the extent ofpenetration 49 is safely away from the spinal canal 50. All of the othertools have positive stops which contact the guide depth stop 51 tocontrol the depth of cut.

The end cut tool 52, shown in FIGS. 11A and 11B, is inserted into theguide 44 to make an end-cut 52, shown in FIG. 11B, for the dovetail.Once completely inserted to the depth stop 53, a single piece interlock54, shown in FIGS. 12A and 12B, which prevented rotation of the blade 55during insertion, is disengaged from the shaft 56 and then preventswithdrawal of the end cut tool 52 from the guide 44. As shown in FIGS.12A and 12B, the interlock 54 is held by spring 57 such that it engagesthe slot 58 in the shaft 56, preventing rotation as shown in FIG. 12A.As the end cut tool 52 is inserted into the guide 44 it pushes theinterlock 54, rotating it out of the slot 58 in the shaft 56 as shown inFIG. 12B. As the interlock rotates, it engages the guide 44 as shown inFIG. 12B. When the shaft 56 is rotated as shown in FIG. 12B theinterlock 54 cannot return to its original position as shown in FIG.12A, thus securing the end cut tool 52 in the guide 44. The rotationinterlock protects the surgeon from the end cut blade 55 and thewithdrawal interlock holds the end cut tool 52 in the guide 44 while theblade 55 is exposed. The surgeon rotates the handle 59 one turn, causingthe end cut blade 55 to make end-cuts 52 as shown in FIG. 11B, in bothvertebrae 47 simultaneously, and returns it to the "zero" position atwhich the end cut tool 52 can be removed from the guide 44.

The dovetail tome 60, shown in FIG. 13A, is inserted into the guide 44to the point where the blade 61 rests against the vertebrae 47. As shownin FIG. 15, the driver 68 is placed on the dovetail tome rod extension62 and drives the dovetail tome 60, cutting the vertebrae 47, until thedepth stop 63 of the dovetail tome contacts the stop 51 on the guide 44,stopping the blade 61 at the end-cut 52, as shown in FIG. 13C. Thedovetail tome blade 61, as shown in FIG. 14A, has endplate breakers 64which split the endplates 65 of the vertebrae (see FIG. 13C) in two 66as shown in FIG. 14B, preventing them from jamming in the blade andpreparing them for later use. The dovetail tome 60 is removed and thebone 67 and the split vertebral end plate 66 contained in the blade 61is harvested for later use in the implant 33.

As shown in FIG. 15, the driver 68 is a pneumatic tool like a miniaturejackhammer. The driver 68 is powered by compressed gas supplied throughthe input tube 69. The driver 68 receives the rod extension from theguide adapter 70 or the rod extension of dovetail tome 62 into a guideport 71. A piston 72, within the driver 68, repeatedly impacts the guideadapter 70 or the dovetail tome rod extension 62, driving the tool intoplace. The driver 68 is activated by the finger-actuated valve 73.Control of the force and rate of the impacts is attained by modulatingthe valve 73. The driver will deliver several thousand small impacts inplace of a few massive blows from a hammer.

The implant 33 of FIG. 5 is prepared for insertion by filling theinterior portion 34 with harvested bone 67 and the split end plates 66from the dovetail tome cuts and additional bone and graft stock. Theimplant 33 is then slid down the guide 44 (FIG. 10) and driven intoplace by the insertion tool 74, shown in FIGS. 16A and 16B. Theinsertion tool 74 has a positive stop 75 which contacts the depth stop51 of the guide 44 and assures correct placement of the implant 33,locking the vertebrae 47.

The above implant devices contain attachment means which are well knownin the woodworking industry, but are not used in Orthopedic SpineSurgery. However, one skilled in the art of intervertebral implantswould readily be able to adapt other fastening devices known in thewoodworking art to spinal implant devices. It should be readily apparentto anyone skilled in the art that there are several available means toattach bone surfaces to the adjacent implant surfaces, such as causingbone anchors to protrude from the implant surface and impinge and attachthe adjacent vertebrae to the implant. Metal staple-like clips can bedriven between adjacent vertebrae to attach the edges of the vertebrae.Tack and staple configurations can substitute for the dovetail tongueand groove fasteners. Bone anchors can also be used to attach naturaltissue to adjacent vertebrae, creating an artificial ligament whichcould scar down, thus retaining an artificial implant within the discspace while osteoinduction takes place and the vertebrae fuse.

We claim:
 1. An implant for mechanically attaching to the ends of andpromoting bony fusion of at least two vertebrae adjacent to a space leftby surgically removed spinal tissue, comprisinga. a load-sharing bodycomprising a structure havingi. a combination of structural elementsfabricated from at least one material which is resected from naturallyoccurring and formerly living tissue, the combination comprising atleast a top surface and a bottom surface; ii. said combination ofstructural elements establishing for the structure as a whole acomposite greater than zero elastic compliance at least in compressionin directions generally axial to said top surface and said bottomsurface; iii. said combination of structural elements further comprisingat least one cavity communicating with both said top surface and saidbottom surface in a configuration suitable as a receptacle for boneimplant and growth material; and b. located on each of said top surfaceand said bottom surface at least one first component of a locking joint,said locking joint comprising at least two components and being capablewhen all components are mated of mechanically anchoring the body to saidadjacent vertebrae so as to transmit tensile and torsional loads to andfrom said adjacent vertebrae, said first component of the locking jointbeing configured to mate with at least a second component of the lockingjoint comprising a geometrically complementary element specially formedin the adjacent vertebra for mating attachment.
 2. The implant of claim1 in which said at least one first component on each of said top surfaceand said bottom surface comprises a dovetail tongue protrusion and saidsecond component comprises a groove of substantially the same shape asthe dovetail tongue protrusion so as to form a locking mechanicaltongue-and-groove joint when the two components are mated.
 3. Theimplant of claim 1 in which each of said at least one first component oneach of said top surface and said bottom surface comprises a horseshoeshaped dovetail tongue protrusion with segments on the outboard portionsof the body, said dovetail tongue shaped so as to mate with said secondcomponent comprising a groove of substantially the same shape as thedovetail tongue protrusion so as to form a locking mechanicaltongue-and-groove joint when the first and second components are mated.4. The implant of claim 1 in which said at least one first component oneach of said top surface and said bottom surface comprises a pluralityof dovetail tongue protrusions, each of which plural dovetail tongueprotrusions is shaped so as to mate with each of said at least onesecond component each second component comprising a groove ofsubstantially the same shape as the dovetail tongue protrusion so as toform a mechanical tongue-and-groove joint when the first and secondcomponents are mated.
 5. The implant of claim 1 in which said at leastone first component on each of said top surface and said bottom surfacecomprises at least one tack device of a tack and staple joint.
 6. Theimplant of claim 1 in which said at least one first component on each ofsaid top surface and said bottom surface comprises at least one boneanchor.
 7. The implant of claim 1 in which the combination of elementscomprises an assembly of at least three modular components, the firstmodular component comprising a right partial separator, the secondmodular component comprising a left partial separator, each having atleast a top surface and a bottom surface, and the third modularcomponent comprising a retainer/keeper.
 8. The implant of claim 7 inwhich the right partial separator and the left partial separator eachhas on each of said top surface and said bottom surface at least onefirst component of a locking joint comprising at least two components,said locking joint being capable when all components are mated ofmechanically anchoring the body to said adjacent vertebrae and therebytransmitting tensile and torsional loads to and from said adjacentvertebrae, said first component of the locking joint being configured tomate with at least a second component of the locking joint comprising ageometrically complementary element specially formed in the adjacentvertebra for mating attachment.
 9. The implant of claim 7 in which theright partial separator and the left partial separator each has on eachof said top surface and said bottom surface at least one protrusion suchthat in concert the protrusions form a first component of a lockingjoint comprising at least two components, said locking joint beingcapable when all components are mated of mechanically anchoring the bodyto said adjacent vertebrae and thereby transmitting tensile andtorsional loads to and from said adjacent vertebrae, said firstcomponent of the locking joint being configured to mate with at least asecond component of the locking joint comprising a geometricallycomplementary element specially formed in the adjacent vertebra formating attachment.
 10. The implant of claim 9 in which the firstcomponent of a locking joint comprises a dovetail shape which fits intoa correspondingly shaped dovetail groove cut in each of the adjacentvertebrae.
 11. The implant of claim 1 in which the top surface and thebottom surface are oriented with respect to one another at an angle suchthat the implant is capable of maintaining spinal lordosis afterinsertion.
 12. The implant of claim 1 in which the at least one materialwhich is resected from naturally occurring and formerly living tissuefrom which the combination of structural elements is fabricatedcomprises human bone allograft material.
 13. The implant of claim 1additionally comprising at least one retractable barb in at least one ofsaid top surface and said bottom surface, said at least one barbretractably engaging the surface of said adjacent vertebra so as toprevent the implant from dislodging.
 14. An implant for mechanicallyattaching to the ends of and promoting bony fusion of at least twovertebrae adjacent to a space left by surgically removed spinal tissue,comprising a single piece of bone allograft material having a top and abottom, said single piece of bone allograft material having at least onedovetail tongue protrusion on each of said top and said bottom formechanically interlocking with said adjacent vertebrae by mating with ageometrically complementary groove in the adjacent vertebra so as toform a locking mechanical tongue-and-groove joint.
 15. The implant ofclaim 14 in which the structure formed from a single piece of boneallograft material is formed into the desired shape and structuralproperties by compression such that the outer layers of the trabecularor cancellous bone form a compressed layer with structural propertiessimilar to those of cortical bone.
 16. The implant of claim 15 in whichthe outer layers of trabecular or cancellous bone forming a compressedlayer comprise a structural shell.
 17. An implant for mechanicallyattaching to the ends of and promoting bony fusion of at least twovertebrae adjacent to a space left by surgically removed spinal tissue,comprising a composite structure fabricated with at least two separateportions of bone allograft material and having a top and a bottom, saidstructure having an internal cavity communicating with said top and saidbottom for receiving autograft bone implant material and bone growthfactors, said unitary structure having at least one dovetail tongue oneach of said top and said bottom for mechanically interlocking with saidadjacent vertebrae by means of tongue-and-groove joints.
 18. The implantof claim 17 in which the at least two separate portions of boneallograft material have different structural properties.
 19. The implantof claim 18 in which the at least two separate portions of boneallograft material with different structural properties are bondedtogether with adhesive to form a composite structure.
 20. The implant ofclaim 17 in which the at least two separate portions of bone allograftmaterial are bonded together with adhesive to form a compositestructure.
 21. The implant of one of claims 20 and 19 in which theadhesive comprises fibrin glue.
 22. An implant for mechanicallyattaching to the ends of and promoting bony fusion of at least twovertebrae adjacent to a space left by surgically removed spinal tissue,comprisinga load-sharing body comprising a structure havingi. acombination of structural elements fabricated from at least one materialwhich is resected from naturally occurring and formerly living tissueand which has a greater than zero elastic compliance, the combinationcomprising at least a top surface and a bottom surface; ii. saidcombination of structural elements establishing for the structure as awhole a composite greater than zero elastic compliance at least incompression in directions generally axial to said top surface and saidbottom surface; and b. located on each of said top surface and saidbottom surface at least one first component of a locking jointcomprising at least two components, said locking joint being capablewhen all components are mated of mechanically anchoring the body to saidadjacent vertebrae and thereby transmitting tensile and torsional loadsto and from said adjacent vertebrae, said first component of the lockingjoint being configured to mate with at least a second component of thelocking joint comprising a geometrically complementary element speciallyformed in the adjacent vertebra for mating attachment.